A technique for building a so-called LAN (Local Area Network) by connecting multiple computers one another with cables has become widespread in offices or the like. Here, in the technique for building a LAN with cables, there are such problems that a construction of cable installation often takes time and cost and in some places or a LAN cannot be built due to the difficulty of cable installation or the like. Furthermore, in recent years, a notebook type personal computer (note PC) convenient for carrying around is often used, yet if the note PC is incorporated into a LAN built with cables, there is a problem that the convenience of portability of the note PC is impaired or the like.
Therefore, recently, there is an increasing demand for a so-called wireless LAN that is built by connecting multiple computers one another via wireless communication.
In the wireless LAN, there are a type that conducts communication via a relay device called as an access point and a type that conducts communication directly between computers without routing any access point. In each type, it is necessary for each computer to mount an antenna device for radio wave communication. Here, it is often the case where wireless communication in this wireless LAN takes place in a space extending in horizontal direction such as within a room or in between rooms adjacent to each other. Therefore, it is desirable for a wireless LAN antenna device used for such a wireless LAN to have stronger directivity in horizontal direction than in vertical direction.
Traditionally, as a technique of obtaining a desired directivity for an antenna device, for example, there have been proposed a technique that designs the shape of an antenna that constitutes an antenna device depending on a desired directivity (See Japanese Patent Application Publication No. Hei 11-122022, for example); or a technique that constitutes an antenna device with an antenna and a passive terminal that has a function of changing a directivity of the antenna and disposes the passive terminal in a position in accordance with a desired directivity. Also, as a technique of obtaining a desired receiving property and a desired transmitting property in an antenna device, for example, there have been proposed a technique that puts a contrivance to the shape of an antenna cover so that the receiving property through the cover toward radio waves coming from above can become a desirable property (See Japanese Patent Application Publication No. 2001-244716, for example); or there have been proposed a technique that puts a contrivance to the shape of an antenna cover in order to make the transmitting property through the cover toward radio waves going upward a desirable property (See Japanese Patent Application Publication No. 2004-15408, for example).